1. Field of the Invention
The present invention relates to a power supply circuit for supplying a direct-current voltage to various electronic apparatuses. Specifically, the present invention relates to a step-down DC-DC converter adopting a switching system.
2. Description of the Prior Art
In recent years, step-down DC-DC converters adopting a switching system (hereinafter, referred to as step-down converters) are used as power supply circuits for many electronic equipments because the step-down converters have high efficient power conversion characteristics. Especially in portable devices, typically a mobile phone, there is a demand for miniaturization, and at the same time, it is strongly demanded to realize large power as the equipments become more sophisticated in functionality. Although the power supplied to a load is relatively small in normal operation, there may be a case where a large current has to be supplied to the load only for a short time, which can be almost an instant. The step-down converter includes an inductor as a main component which is responsible for power conversion. When the inductor reaches magnetic saturation due to the large current, inductance rapidly reduces. As a result, the inductor can not serve as a current-limiting element any more to control a current, which damages a switching element and other components. Meanwhile, in order to prevent the inductor from reaching the magnetic saturation due to the large current, it is required to increase a sectional area of a magnetic core, which increases inductor size.
As an example of inductor used for such a step-down converter, an inductor shown in FIG. 4 has been proposed (for example, see Japanese Laid-Open Patent Publication No. 2000-58344). As illustrated with the inductor of FIG. 4, disclosed herein is such a configuration that the inductor includes: a first core 61 formed of a material which has a high magnetic saturation region and thus hardly reaches the magnetic saturation; a second core 62 formed of a material which has a low magnetic saturation region but has a higher magnetic permeability, the second core 62 being provided in parallel to the first core 61; and a coil 63 which winds on the first core 61 and the second core 62. In this configuration, if a current flowing through the inductor is small, the value of a combined inductance of the inductor increases. Meanwhile, if the current flowing through the inductor is large, the second core 62 reaches the magnetic saturation but the first core 61 does not reach the magnetic saturation, which reduces the combined inductance.
As mentioned in Description of the Prior Art, in the method of structuring the inductor to deal with the large current, the inductance reduces on the occasion of the large current. Therefore, the amplitude of the current flowing through the inductor increases, which increases a ripple voltage superimposed on an output voltage of the step-down converter. Moreover, the size of the inductor increases and the price of the inductor rises.